For many years, mechanical turret tuners have been commonly employed in television receivers to select the VHF channels and a second rotary or continuous tuner has been used to select the UHF channels. For most television receivers, this requires two different channel selection knobs; and the tuners themselves are relatively bulky and require a relatively large amount of space within the television receiver cabinet. Because of the nature of these tuners, it also is necessary to locate them directly behind the front level panel of the receiver, which imposes significant restrictions on the cabinet design and the arrangement of parts within the cabinet, reducing the flexibility of design which would be possible if such tuners could be eliminated.
Some mechanical tuners are equipped with programmable switches to permit them to be used to select either a UHF or a VHF channel at a tuner position by programming the tuner for the local area where the television receiver is to be used. The disadvantages of the cumbersome mechanical tuners, however, are not overcome. Instead, the tuner is made even more complicated by such an arrangement.
It is desirable, and in the U.S. it is becoming necessary, to effect selection of the UHF and VHF channels in a comparable manner. When such tuning compatibility is imposed, significant problems are encountered in providing a mechanical turret-type tuner having detented positions for all of the possible UHF channels which must be accommodated for television receivers capable of operating in any given locality in which the receiver is capable of receiving transmitted television signals. UHF turret tuners with detent tuning selection for each of the 70 possible UHF channels are difficult and expensive to manufacture, and even the display of all of the UHF channel numbers in a manner which is compatible with the display for the much smaller number of VHF channels is difficult to accomplish.
The introduction of voltage-variable capacitor or varactor tuners for the VHF and UHF bands to which a television receiver can be tuned has opened the way for electronic tuning of television receivers. This replaces the cumbersome mechanical turret tuners and allows greater flexibility in the design of the channel selection panel and in the location of tuner parts within the receiver cabinet. Even so, if the receiver is to be made capable of individual selection of any one of the 70 UHF channels in addition to the VHF channels, it has been necessary to provide a large number of individual tuning components. For example, in many prior art electronic tuner control circuits, it has been necessary to provide a separate tuning potentiometer for each of the 70 UHF channels if full capability of UHF channel selection is desired. This results in a relatively expensive tuner configuration requiring a large number of parts.
As a compromise, the number of UHF channels to which any individual receiver can be tuned generally is reduced to a number comparable to the number of VHF channels. Then, when the receiver is placed in operation in a given locality, the UHF channels in that locality are tuned by selected ones of the available UHF potentiometers. A number or some other indicia is placed on the display portion of the tuner control panel to indicate the number of the particular UHF channel which thereafter is to be selected at that position by the tuner control. Of course, if the receiver later is moved to another locality, this necessitates retuning of the UHF channels and also means that a change in the indicia on the tuner control panel must be made. This clearly is not an optimum solution to providing compatible tuning of UHF and VHF channels.
It is desirable to provide a television tuning control system which is capable of tuning to any channel which the receiver can receive with equal ease of selection of VHF or UHF channels. In addition, it is desirable to provide a tuner control system which uses a minimum number of parts and which does not require re-programming when the receiver is moved from one locality to another.